A fuel cell system capable of carrying out highly-efficient, small-scale electric power generation has been expected as a distributed power generating system capable of realizing high energy use efficiency, since it is easy to configure a system for utilizing heat energy generated when a fuel cell generates electric power.
In the electric power generating operation of the fuel cell system, a hydrogen-containing gas and air (oxidizing gas) are supplied to a fuel cell stack (hereinafter simply referred to as “fuel cell”) provided as a main body of an electric power generating portion of the fuel cell system. Then, an electrochemical reaction using hydrogen contained in the hydrogen-containing gas supplied to the fuel cell and oxygen contained in the air supplied to the fuel cell proceeds in the fuel cell. By the progress of the electrochemical reaction, chemical energies of the hydrogen and the oxygen are directly converted into an electric energy in the fuel cell. Thus, the fuel cell system can output electric power to a load.
Here, a system for supplying the hydrogen-containing gas necessary during the electric power generating operation of the fuel cell system is not developed as an infrastructure. Therefore, a conventional fuel cell system is provided with a hydrogen generator configured to generate the hydrogen-containing gas necessary during the electric power generating operation. The hydrogen generator includes at least a reformer. By the progress of a steam-reforming reaction in a reforming catalyst body provided in the reformer, the hydrogen-containing gas is generated from the raw material, such as a city gas containing an organic compound, and water. In this case, the reforming catalyst body of the reformer is heated by a suitable heating device to a temperature suitable for the progress of the steam-reforming reaction. For example, since the heating device (burner, or the like) can combusts a mixture gas of the city gas and the air, the reforming catalyst body of the reformer can be heated by a high-temperature flue gas. In addition, in the electric power generating operation of the fuel cell, an anode off gas unconsumed in the fuel cell can be combusted in the above-described burner. Thus, the reformer having been heated to have a suitable temperature can efficiently generate the hydrogen-containing gas by the reforming reaction between the raw material, such as the city gas, and the steam.
The steam is generated by using a water evaporator provided in the hydrogen generator and is used in the reforming reaction of the reformer.
Moreover, while the fuel cell stops operating, input portions and output portions of gases (the raw material, the hydrogen-containing gas, and the oxidizing gas) and reforming water are sealed to prevent gas passages of the hydrogen generator and reactant gas passages of the fuel cell from being communicated with the atmosphere. By sealing these portions, it is possible to prevent outside air from getting into the fuel cell and the hydrogen generator.
Meanwhile, with the input portions and the output portions completely sealed, an internal state of the fuel cell system may become an excessive positive pressure state or an excessive negative pressure state with respect to the atmospheric pressure.
Especially, in a case where the communication between an internal space of the hydrogen generator and the outside air is blocked while the hydrogen generator stops operating, i.e., in a case where a sealed state of the hydrogen generator is realized while the hydrogen generator stops operating, an excessive pressure applied state of the hydrogen generator may occur by a volume expansion caused due to water evaporation in the water evaporator. Here, by open-close control of, for example, a solenoid valve for sealing, the inside of the hydrogen generator is temporarily open to the atmosphere to depressurize the inside of the hydrogen generator (see Patent Document 2 for example).
Specifically, Patent Document 2 (for example, paragraph 0039) describes a method in which: a controller of the hydrogen generator detects the increase in the internal pressure of the hydrogen generator; and if the internal pressure abnormally increases, an on-off valve provided downstream of the reformer is temporarily open to discharge an internal gas of the hydrogen generator to an outside of the hydrogen generator.
Moreover, in a case where the temperature of the hydrogen generator is decreased after the sealed state is realized, and this causes the negative pressure state, a predetermined amount of the raw material is forcibly supplied to the inside of the fuel cell system to pressurize the inside of the fuel cell system.
These depressurizing and pressurizing operations are hereinafter referred to as a pressure keeping operation of the hydrogen generator. By the pressure keeping operation, the operation of the hydrogen generator can be appropriately stopped while preventing the internal pressure of the hydrogen generator from being applied to devices, i.e., maintaining the internal pressure of the hydrogen generator at an appropriate level.
In a case where power supply to the hydrogen generator is cut by power outage or the like during the operation of the hydrogen generator, and this stops the operation of the hydrogen generator, the increased internal pressure of the hydrogen generator cannot be released to the atmosphere by the method described in Patent Document 2.
Here, Patent Document 1 proposes a fuel cell system in which a water sealing mechanism is provided on a passage by which the hydrogen generator and the heater are communicated with each other.
In accordance with the fuel cell system described in Patent Document 1, when the fuel cell system normally stops, the water sealing mechanism can seal the inside of the fuel cell system (hydrogen generator). In contrast, in a case where the internal pressure of the hydrogen generator is increased to a predetermined pressure or higher by the water evaporation at the time of the power outage, water sealing of the water sealing mechanism by a water head difference is automatically lost, and the internal gas can be discharged to the outside without the power supply. With this, the internal pressure of the hydrogen generator at the time of the power outage can be maintained at an appropriate level, and failures of the devices by the increase in the internal pressure of the hydrogen generator can be prevented.
Patent Document 1: Pamphlet of International Publication WO 2006/013917A1
Patent Document 2: Japanese Laid-Open Patent Application Publication 2005-243330